1. Field of the Invention
The present invention relates to an optical masking apparatus of an OTDR.
2. Prior Art
In a measurement of insertion loss of an optical fiber using an optical time domain reflectometer (OTDR), a reflected light through Fresnel lens (hereinafter referred to as a Fresnel reflected light) and a backward scattered light caused by a Rayleigh scattering are detected, thereby evaluating an insertion loss characteristic of the optical fiber in the longitudinal direction thereoff
In this case, if the backward scattered light caused by Rayleigh scattering and a Fresnel reflected light in an optical fiber passage are measured at the same time by the OTDR, there is a large difference therebetween in level so that the backward scattered light can not be accurately estimated due to the influence by the Fresnel reflected light. As a result, accuracy of the measurement is deteriorated and a part of the optical fiber passage forms an area where the measurement can not be performed, which leads to the greatest drawback of the measurement by the OTDR.
To overcome such a drawback, there has been employed a so-called optical masking method for removing the Fresnel reflected light using a high-speed optical switch, thereby removing unnecessary reflected light. A conventional optical masking apparatus will be described with reference to FIG. 6.
An optical pulse generated by a pulse light source 11 is introduced into a high-speed optical switch 12 from the side A to side B thereof and it is irradiated from an optical output terminal 20 to an optical fiber 10 to be measured. Backward scattered light which is a part of a Fresnel reflected light or a Rayleigh scattered light from the optical fiber 10 to be measured is returned to the high-speed optical switch 12 by way of the optical output terminal 20. The returned light is introduced into the high-speed optical switch 12 from the side B to side C thereof.
At this time, the high-speed optical switch 12 may be switched to the side A for preventing unnecessary optical signal from being input to an optical/electric transducer 15 for realizing the optical masking. In this case, the high-speed optical switch 12 is operated in synchronization with the optical pulse which is generated by the pulse light source 11.
FIG. 7 shows a waveform of the OTDR which is displayed on an indicator 19.
In the optical masking apparatus of the OTDR as shown in FIG. 6, the insertion loss is large since the high-speed optical switch is used for optical masking. As a result, a dynamic range which is an important property of the OTDR is deteriorated. Moreover, alignment of optical axes in the high-speed optical switch is very difficult and troublesome. Furthermore, the high-sped optical switch is normally very expensive, which increases the cost of the OTDR.